Since C. W. Tang et al. of Eastman Kodak Company invented organic EL elements achieving high luminance (Appl. Phys. Lett., Vol. 51, page 913, 1987) in 1987, there have been rapid progresses in the development of materials for the organic EL elements and in the improvement in element structures. Practical uses of the organic EL elements have started recently in, e.g., displays of car audio systems and cellular phones. In order to further expand the uses of the organic EL (electroluminescence), the development of materials that provide improved luminous efficiency and durability and the development of full color displays, etc. are currently underway in earnest.
The organic EL element requires a pair of electrodes and a luminescent layer held by these. Between an anode and the luminescent layer, an anode buffer layer (also called a hole injection layer or the like) is optionally further provided adjacently to the anode, for the purposes of, e.g., decreasing the voltage to drive the organic EL element, enhancing the current efficiency, stabilizing the hole injection to enhance the electrical power efficiency (∝luminous efficiency/driving voltage) of the organic EL element in order for the organic EL element to have a longer life, and coating a projection of the anode and the like to decrease a defect of the element.
As a material for the anode buffer layer, there can be mentioned, for example, a polymer material and an oligomer material, and various materials have been known.
As the polymer material, for example, polyaniline, polythiophene (e.g., polyethylene dioxythiophene (PEDOT)), and a material that is made conductive by mixing an oxidant into a polymer having arylamine as a main chain or a side chain are known (Non-Patent document 1).
For example, Patent document 1 (JP-A-2000-36390), Patent document 2 (JP-A-2001-223084) and Patent document 3 (JP-A-2002-252085) each describe a hole injection layer comprising an electron-accepting compound (an oxidant) and a polymer that has a specific arylamine structure. A method for forming the hole injection layer is described therein as including the steps of preparing a coating solution containing the electron-accepting compound and the polymer that has a specific arylamine structure, coating an anode with the coating solution by means such as spin coating and dip coating, and drying the solution.